I'm not personally offended by the Tshirts and I'm not sure I know anyone who would be.
I can see the airline's point about children seeing bad language, however I also find myself unable to watch much of the TV during the day / even leave a newspaper lying around when DC are there due to bad language / adult themes, so I think the Tshirts are pretty mild by comparison. I'm far more relaxed about having to explain the word 'bitches' than having to explain a newspaper headline about a paedo or a suicide bomber.
I think it's reasonable to ask the women to turn the Tshirts inside out or cover them up for the duration of the flight while out of their seats. I'm pretty sure if it was made clear it was this or book new flights they'd have complied.